1. Field of the Invention
The present invention relates to the field of liquid crystal display, and in particular to an LED (Light-Emitting Diode) backlight drive method and drive circuit of liquid crystal display.
2. The Related Arts
Liquid crystal displays (LCDs) have a variety of advantages, such as thin device body, low power consumption, and being free of radiation, and is thus widely used. Most of the LCDs that are currently available in the market are backlighting LCDs, which comprise a liquid crystal panel and a backlight module. The operative principle of the liquid crystal panel is that liquid crystal molecules are interposed between two parallel glass substrates and the liquid crystal molecules are controlled to change direction by application of electricity to the glass substrates in order to refract out light emitting from the backlight module for generating images. Since the liquid crystal panel itself does not emit light, light must be provided by the backlight module in order to normally display images. Thus, the backlight module is one of the key components of the LCDs. The backlight module can be classified in two types, namely side-edge backlight module and direct backlight module, according to the position where light gets incident. The direct backlight module comprises a light source, such as a cold cathode fluorescent lamp (CCFL) or a light-emitting diode (LED), which is arranged at the back side of the liquid crystal panel to directly provide a planar light source to the liquid crystal panel. The side-edge backlight module comprises a backlight source comprising an LED light bar that is arranged at an edge of a backplane to be located rearward of one side of the liquid crystal panel. The LED light bar emits light that enters a light guide plate (LGP) through a light incident face at one edge of the light guide plate and is projected out of a light emergence face of the light guide plate, after being reflected and diffused, to transmit through an optic film assembly to form a planar light source for the liquid crystal display panel.
Referring to FIG. 1, which is a conventional LED backlight drive circuit for a liquid crystal display having 2D and 3D modes, a constant current driving IC (constant current driving chip) 300 comprises an OVP pin (providing overvoltage protection) that is associated with an internal voltage comparator 200 in connection with a divided voltage of a driving voltage of an LED string 100 obtained with a series connection of resistors R11, R12, R13. When the voltage across the resistor R13 is greater than a constant voltage source (which is generally 2V) internally of the constant current driving IC 300, the constant current driving IC 300 cuts off a driving signal to a field effect transistor so that an output voltage (which is the driving voltage of the LED string 100) is no longer increased thereby protecting the components of the backlight drive circuit. The electrical current flowing through the LED string 100 is of a linear relationship with respect to the desired level of voltage. In a liquid crystal display having 2D and 3D modes, the peak value of LED driving current for the backlight source in the 3D mode is high so that the level of the driving voltage is also high. Taking a string having 8 LEDs as an example for computation purposes, the driving voltage required for 3D mode is about 10V higher than that for 2D mode. Consequently, design of the voltage level for overvoltage protection is generally done as 1.2 times of the driving voltage of the LED string in 3D mode. On the other hand, if the voltage level for overvoltage protection is designed according to the required driving voltage for the LED string 100 in 2D mode, the driving voltage may not be sufficient for the LED string 100 in 3D mode, making it not normally lit.
However, using the driving voltage required for LED string in 3D mode to design the voltage level for overvoltage protection has the following shortcoming. When abnormality occurs in the operation in 2D mode, the output voltage is excessively high, which causes excessive instantaneous impact to the components of the backlight drive circuit, thereby shortening the lifespan of the components.